In the past it has been known to use switching type preregulators to provide DC power, for example, to electronic ballast circuits for fluorescent lamps. Such preregulators include, but are not limited to a boost-type topology, and have also been known to use input power factor control circuits which have become so popular and in such demand that such power factor control circuits have been reduced to integrated circuit form. One such integrated circuit which has been found desirable for such application is manufactured by Siemens AG under model designation TDA 4817. This type of integrated circuit, or "IC", is intended to be used with a power semiconductor switch, typically a MOSFET transistor, inductor, diode and output capacitor. Such IC's have a current envelope input useful in improving the power factor of electronic power supplies, including those for powering fluorescent lamps. However, such arrangements have been observed to exhibit distortion in the region of the zero-crossing of the AC line sine wave power source. This crossover distortion is particularly significant at low power levels. It has been found to be caused by the non-linearity in current transfer resulting from the effect of charging four inherent (or "parasitic") capacitances in such circuits. The four parasitic capacitances are the winding capacitance of the inductor, the output capacitance of the transistor, the junction capacitance of the diode, and the stray capacitance of the circuit. In an ideal circuit, turning off the transistor will result in a step increase in voltage across the transistor, allowing the diode to immediately begin conduction of current from the inductor to the load. However, in an actual circuit, the inductor current will charge the four parasitic capacitances giving a finite slope to the rising voltage across the transistor as it turns off. When the AC line voltage is near the zero crossing, the inductor current becomes small enough that the energy stored in the inductor is not sufficient to charge the four parasitic capacitances to the voltage required to cause the diode to conduct. The resonant circuit formed by these four parasitic capacitances and the inductor oscillates until the transistor is turned back on. Under this condition, no power is delivered to the output capacitor and the load, and hence, very little current is drawn from the AC line. When the load is large, the inductor current is large enough that the four parasitic capacitances are quickly charged and crossover distortion is less significant. However, with light loads, crossover distortion is more pronounced.
It has been found that, for a fixed load, adding a bias at the current envelope input to the power factor control integrated circuit will provide compensation to reduce crossover distortion, but with varying loads a fixed bias results in unacceptable performance except at the particular load for which the bias was designed since the compensation will not be correct as the load varies from the fixed design point.